Hi,
It is very likely a defective tube.
Unless you just want to buy a whole new set, the conventional approach is to swap tubes from one amp to the other until you find it. First swap half the tubes, maybe output tubes. If the problem moves it is in one of those tubes. If it does move, swap half of those back to the other amp and so on until you have identified the culprit. Divide and conquer!
You could also try turning off the lights in the room and just observing the tubes when the noise occurs. It is possible you will be able to see the bad tube arcing.
Good Luck. BTW, my neighbors are from Mauritius.
It is very likely a defective tube.
Unless you just want to buy a whole new set, the conventional approach is to swap tubes from one amp to the other until you find it. First swap half the tubes, maybe output tubes. If the problem moves it is in one of those tubes. If it does move, swap half of those back to the other amp and so on until you have identified the culprit. Divide and conquer!
You could also try turning off the lights in the room and just observing the tubes when the noise occurs. It is possible you will be able to see the bad tube arcing.
Good Luck. BTW, my neighbors are from Mauritius.